This invention relates to a process for forming a phosphor powder layer, more particularly to a process for forming a phosphor powder layer used as a scintillator.
Heretofore, as a radiation detector used in X-ray CT (computerized tomography), there has been used a detector combining a xenon gas chamber or bismuth germanate with a photomultiplier. But when a large number of radiation detectors having equal performance were necessary as in the case of X-ray CT, it was difficult to adjust characteristics of individual radiation detectors in the case of using such detectors. Particularly in the case of a radiation detector combining bismuth germanate with a photomultiplier, it was very difficult to adjust characteristics of individual radiation detectors due to dispersion in characteristics of monocrystals of the bismuth germanate used as scintillator and dispersion in characteristics of the photomultipliers.
In order to solve this problem, some of the present inventors previously proposed a radiation detector in which phosphor powders (particles) were used as scintillator (U.S. patent appln. Ser. No. 047,133, filed June 11, 1979, now U.S. Pat. No. 4,317,037). For the purpose of obtaining a tomography of high accuracy in a radiation detector for conventional X-ray CT, the width of scintillator is about 1-10 mm and preferably about 1-3 mm and the length thereof is about 20 mm, for example. Accordingly, the number of phosphor particles in one radiation detector is about 300,000, though it may vary depending on the particle size. Although individual phosphor particles may possibly be slightly different from one another in characteristics, the dispersion in the characteristics as scintillator can be made about one divided by the square root of particle number, that is, about 0.01%, by sufficiently mixing them, whereby a satisfactory result can be obtained. A radiation detector resembling to the above-mentioned one is also disclosed in Japanese Patent Appln. Kokai (Laid-Open) No. 90089/79.
As a process for forming a thin layer of phosphor powder, there has been employed, for example, a process wherein a phosphor powder is suspended in a polymer solution, and the resulting suspension is placed in a mold having a desired shape and then dried. But this process had defects in that voids are easily formed in the resulting thin layer and a packing density of the thin layer cannot be increased. In the case of another process wherein a monomer solution was used in place of the polymer solution and the polymerization reaction was conducted at the time of molding, said process had almost the same defects as mentioned above as well as other defects such as a much more time being necessary for the polymerization reaction, the desired shape used in a radiation detector being not able to be formed in a mold from the beginning, and a large block having to be formed first, followed by cutting and polishing to give the desired shape, which results in making the cut and polished face lower in emission efficiency. There has also been employed a process wherein a phosphor powder and a polymer powder are mixed, charged in a molding machine and formed into a powder layer with heating and/or under pressure. This process was considerably good when a mixing ratio of the phosphor to the polymer was small, but when the mixing ratio of the phosphor to the polymer became large in order to increase the packing density of the phosphor powders, a uniform mixture could not be obtained due to a large difference in specific gravities of the two, which results in unfavorably giving ununiform molded articles having high polymer contents locally and having some weak portions. It has also been proposed a process wherein a phosphor powder was fired to give a thin film, but when a phosphor, Gd.sub.2 O.sub.2 S:Pr,Ce,F, which is suitable as scintillator, was fired in air, emission efficiency of the resulting thin layer was lowered undesirably.